Sorry, but I don't have my copy of how long it should take to get over the fear of <<whatever it is you're scared of>>
I expect that is because there are no rules about that and it can only be done at your own pace. It's a natural response and nothing to be ashamed of or embarrased about.
I don't know how you came off in the first place and that really puzzles me. I can only guess that you took it very slow and lost your front wheel, you took it on a bend when it was wet and lost it, you didn't hit it square on or there was a groove or rail and you tramlined it and lost it that way.
I prefer to take cattle grids fast (up to 40mph, hill permitting) than slow, but I pick my line before I go over it. If I'm behind a group I often slow down well before the grid just so I can sppeed up and ride over it a bit faster. Firstly, you get a smoother ride over the grid and secondly, if your wheel does start to slide (it hasn't happened to me yet) then extra speed will give extra stability and also give you a better chance of reaching tarmac where you can regain tyre grip and control. I also don't brake on cattle grids. If I need to slow down, I slow down a bit extra vefore the grid and let go of the brakes to ride over the grid. Braking on metal, especially wet metal, is a recipe for skidding.
I think those New Forest grids have rails which can tramline your wheels. The late Dave Lewis had that on an event in the New Forest over 10 years ago and was out of action for a while because of that.
I also check for a big gap or missing rung, especially the first and last. I sometimes jump if there is a missing rung. Not just missing rungs, sometimes the road erodes from the first or last rung which can mean a big gap entering or exiting the grid and possibly ruin your wheels.
If I'm going less than about 8mph, I usualy get off and walk, especially on steep hills in the wet where trying to grind uphill on wet metal could lead to me wheelspinning, stalling and having my front wheel spin around and fall down the gap.
It's similar to mountain biking, it's part confidence and part ability. The trick is to not overestimate your ability (you're easily capable of riding over a cattle grid if you know what the dangers are) but also not to lose confidence. Either do it or don't do it.
DO NOT go into it undecided. When I go mountain biking I always take the cowards option if I'm not entirely sure and I build up my confidence bit by bit.
If you're approaching a grid, then decide well before you get there if you're going to ride over it or not. DO NOT decide to ride over it, then change your mind at the last minute! It might end up with you doing an emergency stop and not stopping until you hit the grid, which probably wouldn't end well.
Following someone else over a grid may help build up your confidence. Starting with well made grids (some are smoother to ride over than others and don;t have grooves or rails)
Just don't feel bad about walking over them and don't put pressure on yourself to be able to ride over them.
I remember crossing a metal grid on an American 1200km ride one wet night at about 25mph and losing my front wheel twice and sliding all over the place. How I never came off, I don't know but when I finally crossed the metal and reached tarmac I was on the wrong side of the road.
I walked over it on the way back, then was spotted by the police and asked if I was OK.
